What time are you testing your ph, in relation to your photoperiod?
What is the ALK?

First try increasing gas exchange with the use of a powerhead agitating the surface of the water or a skimmer. I have no opinion on the best buffer, most are all the same anyway. Just make sure you buffer what needs buffering, there is ph buffer and alk buffer, while they are related they are not the same.

Im testing ph at different times - lights on lights off - using 40w no till 12/10. photoperiod is 12 on 12 off. i have kent marine superbuffer dkh. 75gal 10g sump MJ 1200 and return at surface 2 penguin 550's for the rest

Try increasing gas exchange as already stated with a powerhead and also trying to get the surface to come into contact with fresher air. Sometimes air indoors can get "stale" and have a high CO2/O2 ratio. I've been successful in raising my tank pH just by leaving the canopy off and opening a window. While effective sometimes not always practical (its 30 degrees F outside right now) Then you might have to fall back on a buffer. Just make sure you know your carbonate levels etc before choosing...some additives will raise pH and alkalinity some will just raise one or the other. Also, when you upgrade your lighting you may see a rise in pH as stronger lighting=more photosynthetic activity=CO2 used up and O2 produced.